creates a new random value for the coefficient
Parameters:
Bool_t near : takes a random value near the current value
double value : this is the current value
double spread : the sigma of the gaussian which is taken to calculate the new value
Bool_t mirror : if the new value would be outside of the range, mirror = false
maps the value between the constraints by periodic boundary conditions.
With mirror = true, the value gets "reflected" on the boundaries.